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Date: Wed, 28 Aug 2002 17:04:33 +0200
From: Frank.Otto@tc.pci.uni-heidelberg.de
Message-Id: <200208281504.g7SF4Xl04292@goedel.pci.uni-heidelberg.de>
To: linux-kernel@vger.kernel.org
Cc: mdheffner@yahoo.com
Subject: Re: PROBLEM:  conflict between apm and system clock on Inspiron 8100
In-Reply-To: <1030381725.1750.10.camel@irongate.swansea.linux.org.uk>
References: <20020826170037.69164.qmail@web40210.mail.yahoo.com> <1030381725.1750.10.camel@irongate.swansea.linux.org.uk>
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Alan Cox wrote:
> On Mon, 2002-08-26 at 18:00, mike heffner wrote:
> > Well, isn't that a nice feature.  Is there a
> > workaround for this hardware?
>  
>  A thinkpad ;)

Unfortunately, that's not true -- I just got an IBM Thinkpad R32
which exhibits the same behaviour as Mike's Dell Inspiron 8100,
it's only a tad worse. When I have the battstat_applet running (which
checks the battery every second), kernel time runs about 3% slow
compared to the RTC (which seems to be half-way accurate on my machine).

The cause seems to be definitely APM. If I shut off battstat_applet
and apmd, kernel time and RTC are in sync. With only apmd, I lose about
15 seconds per hour. With battstat_applet, I lose 2 minutes per hour.
With
  while true; do cat /proc/apm >/dev/null; done
the system runs at about 1/4 of the right speed. Using a kernel with ACPI
eliminates the problem (of course, you lose almost all power management
functionality too).

BTW, I have set CONFIG_APM_ALLOW_INT, and on startup the kernel even says
"IBM machine detected. Enabling interrupts during APM calls." Doesn't
seem to help, though.

Alan Cox continues:
>  In theory you could try writing some code to measure the elapsed time by
>  other means and then correct the kernel for the number of lost ticks.
>  Not trivial. Or for that matter dont run battstat

I've hacked together a small daemon that tries to adjust the value
and speed of the kernel clock (via adjtimex) to the RTC. An ugly solution,
I know, but better than nothing. If anyone is interested, mail me.

Regards,
Frank

-- 
Please CC replies to me since I'm not on the list.
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